Have you ever wondered where horses come from? Have you ever wondered why they look and act as they do? Wondered why they have chestnuts on their legs and why they run so fast? To answer these questions we need to travel back 50 million years. For an equine professional, a look back might give insight into how to better understand and care for this unique animal.
Luckily for us, the evolutionary history of today’s modern horse (Equus Caballus) is well known. Thanks to the horse’s tendency to live in herds, it has been buried and fossilized in large numbers, making its evolutionary history more clear than most other species. Also, many of the fossils discovered were located right here in North America. Remains of the first species in the ‘hippus’ (horse) family, Eohippus, were found in the Wasatch Range in Utah and in the Wind River basin in Wyoming (1870). While there are some intermediate and overlapping species throughout the long history of this creature, there are four commonly-mentioned species that lead us to today’s Equus.
The origins of the horse can be traced back more than 50 million years to the “dawn horse” (Eohippus or Hyracotherium). It was about the size of a fox (about three hands tall) with a short neck, long hind limbs and arched flexible backbone. Some authors claim that this horse may have been nearly as fast as the modern racehorse. Eohippus had very slender limbs, with four hooved toes on its fore feet and three on its hind. While the weight was carried on the central pad, like that of a dog’s foot, the toes were there to prevent the creature from sinking into the soft boggy earth of the hot, swampy jungles in which it lived. Its teeth were suitable for browsing on leaves from the swamp cypress and mammoth trees.
As the climate began to change and the ground hardened, this creature began to change as well. The Mesohippus, which lived 35 million years ago, was about twice the size of the dawn horse. It weighed around 70 pounds and was about six hands tall. Though its diet still mainly consisted of leaves, you start to see the development of the gap between the front and back teeth typical of grazing animals. The feet now had three toes on both the front and back legs with a prominent central toe.
Merychippus lived about 25 million years ago and marks an important turning point in the animal’s anatomy. This creature migrated to the steppes during the Miocene period, changing both its body and diet. It was also larger—about 10 hands—and made use of only the central hooved toe (the central toe was strengthened due to the firmer ground and the side toes were reduced). Also, the teeth had now been adapted for the hard grass of the steppes rather than a leaf diet. As the temperatures dropped during this period, conifers outnumbered deciduous trees and grass became more readily available as the ground solidified. Another key change brought about by these environmental transformations was the eye socket’s position, which was altered for better vision due to their new grazing habits.
Next came the Pliohippus, which lived during the Pliocene period about 10 million years ago. This animal had a body equipped for survival in savanna-type conditions. The skull and jaw continued deepening, while the higher crowned molars developed for grinding the grass. This was the first single-hoof equine with no recognizable side toes (they developed into the chestnuts we know today). It stood at about 12 hands and had a skeletal structure that enabled it to graze freely, roam the plains and run swiftly from enemies.
Within the last six million years we see Equus, which has developed into our modern-day horse. And about 5,000 years ago the horse was first domesticated.
The large strapping animals we know today are a far cry from the three-hand-tall, four-toed animal that roamed the earth so many years ago. But, by following its evolution, we can see why our modern-day friends sometimes act the way they do. Perhaps the next time a horse spooks and runs, we should remember that it wasn’t so long ago that it was running for its life on the open plains.